Data communication systems using an infrared ray are classified as follows.
Type 1: A transmission apparatus of directivity radiates an infrared ray of narrow beam width, and a receiving apparatus of narrow view angle directly receives the infrared ray.
Type 2: The transmission apparatus of directivity or a transmission apparatus of indirectivity radiates an infrared ray of wide beam width, and a receiving apparatus of wide view angle directly receives the infrared ray.
Type 3: The transmission apparatus of directivity radiates an infrared ray of narrow beam width, and a receiving apparatus of wide view angle directly receives the infrared ray.
Type 4: The transmission apparatus of directivity or the transmission apparatus of indirectivity radiates an infrared ray of wide beam width, and the receiving apparatus of narrow view angle directly receives the infrared ray.
Type 5: The transmission apparatus of directivity radiates an infrared ray of narrow beam width, and the receiving apparatus of narrow view angle receives the infrared ray reflected by the ceiling or the wall of the room.
Type 6: The transmission apparatus of directivity or the transmission apparatus of indirectivity radiates an infrared ray of wide beam width, and the receiving apparatus of wide view angle receives the infrared ray reflected by the ceiling or the wall of the room.
Type 7: The transmission apparatus of directivity radiates an infrared ray of narrow beam width, and the receiving apparatus of wide view angle receives the infrared ray reflected by the ceiling or the wall of the room.
Type 8: The transmission apparatus of directivity or the transmission apparatus of indirectivity radiates an infrared ray of wide beam width, and the receiving apparatus of narrow view angle receives the infrared ray reflected by the ceiling or the wall of the room.
The types 1, 2, 3, 4 are called line of sight (LOS) link type communication system because a receiving side directly receives the infrared ray radiated by a sending side. The types 5, 6, 7, 8 are called a non-line of sight (LOS) link type communication system because the receiving side does not directly receive the infrared ray radiated by the sending side. As a condition of the view type communication system, the transmission side and the receiving side exist in a view area line of sight. If an obstacle object exists between the transmission side and the receiving side, the communication is not executed. On the other hand, the non-view type communication system uses a diffusion light reflected by the ceiling or the wall. Therefore, even if the transmission side and the receiving side are not located in the view area, the communication is executed. Especially, the type 6 is called an infrared ray communication system of diffusion type, whose free degree of communicatable device location is largest among the above eight types.
However, in the infrared ray communication system, it is not always assured that a particular device (own terminal) can communicate to all other devices surroundingly existed. In short, a hidden terminal to which the direct light and the reflected light do not reach often exists. For example, naturally, the particular device can not communicate to other devices spaced more than the maximum communicatable distance away. It is possible that a neighboring device becomes the hidden terminal because the infrared ray is not transmitted by the obstacle object. Furthermore, in case a plurality of devices are located to mutually communicate, all devices must be located in a common area in which communicatable areas of the all devices overlap. If at least one device is not located in the common area, mutual communication is not executed.
FIGS. 1A and 1B show an example of mutual communicatable area for a plurality of devices. In FIGS. 1A and 1B, a communicatable area of the infrared ray communicatable device 91 is 910, a communicatable area of the infrared ray communicatable device 92 is 920, a communicatable area of the infrared ray communicatable device 93 is 930. In order for the devices 91, 92, 93 to mutually communicate, the communicatable areas of the devices 91, 92, 93 must overlap. As a result, a mutual communicatable area is limited. In addition to this, if another device 94 appears, the other device 94 must be located so that all communicatable areas of the devices are overlapped. If the device 94 is located as the communicatable area 940 shown in FIGS. 1A and 1B, mutual communication of four devices 91, 92, 93, 94 is impossible.
In FIGS. 1A and 1B, the communicatable area of each device is represented as a circle or a rectangle. However, actually, the communicatable area is transformed by direction of the sending apparatus and effect of the obstacle object. In proportion to increase of a number of devices, the mutual communicatable area is further limited.
As a method to extend the mutual communicatable area, an infrared ray repeater including a transmitter and a receiver for the infrared ray is used. This infrared ray repeater has a function to reflect a light signal and retransmit the received light signal after amplification. However, even if the infrared ray repeater is used, the infrared ray does not have a transparency of substance such as a wireless wave of ISM (Industrial Scientific Communication) band, and its ability of diffraction is low. Therefore, it is difficult that the infrared ray repeater is located at communicatable position for all devices in comparison with a repeater of a wireless wave. At a place where the infrared ray repeater is not located at communicatable position for all devices, it is difficult that the mutual communicatable area is extended by using the infrared ray repeater.
Furthermore, in case N devices set communication paths of connection type to mutually communicate, N(N−1)/2 connections must be set and each device must manage (N−1) connections. As a result, the processing load to manage the connections increases in proportion to the increase of the number of devices.
Even if one device can communicate to other devices to which the light does not directly reach by using the infrared ray repeater, each device must mutually set the connections through the infrared ray repeater. In order for N devices to mutually communicate, N(N−1)/2 connections must be set and managed. Therefore, the processing load to manage the connection increases in proportion to the increase of the number of devices.